Thanks for that.  What had me confused was that I read that the 75mm production had been completely stopped, so, at some point, the 76mm guns would be used mainly used in the same infantry support role as its predecessor.  It seemed logical that a comparable HE round would be needed.   I am aware that the cannon was designed to use the same projectiles as the 3", but my understanding is that even the tank destroyers using 3" ammo shot a lot of HE and would have preferred a more effective round.

Maybe the sheer amount of surplus ammo from WWII production led to the "it's good enough" decision.

The earlier 76mm guns like the M1A2 in the Sherman stayed around for a long time, although largely used by other countries.   Do you know if any improved HE round were developed for or by them?  Could the M352 used by the M41 tank  be fired by the older cannons? 

Perhaps having the AP and HE rounds have the same velocity was a factor, making aiming simpler.

That might be the case, but the HVAP round that appeared in late 1944 had a different velocity than the previous AP and HE, so I doubt that wanting to match velocity was the only issue.  I am starting to lean towards the "it's already in production and it's good enough"  school of thought.

As Kocur noted, the 76mm M32 gun used in the M41 tank did indeed have the M352 HE shell that had a lower velocity than the AP and HVAP and pretty much matched the old 75mm shell's explosive content.  This indicates a change of heart for one reason or another after WWII.

Still a mystery, I guess.  The Army did many silly things in the war, like not training the troops in North Africa how to use a bazooka before the were deployed to the front and not teaching tankers how to use the gyro-stabilizer, which was very effective for trained crews and not at all for the untrained.  They also cut a foot off the ww2 76mm gun instead of adding weight to the back of the M18's turret.

I can't see any mystery. M42A1 was US Army standard  3'' HE shell (apart from tank guns it was used in AA guns and seacoast guns) and the only one in production. 

That it was substantially less effective than 75 mm M48 against ground target was really noticed only after Normandy, when 3''/76 mm guns in GMCs and tanks saw wider use. There was not enough time to replace it before end of WW2. 

Even if it was noticed earlier, better shell would have faced a lot of competition from other weapons  programs.

Apart from manufacturing issues, often cited reason to develop US first electronic computer was to speed up calculations for firing tables for all the new ammunition being developed during WW2. I mean, that too apparently was a bottleneck. 

That's all speculation.  It's a mystery because none of us here have produced any documentation on the decision.  We can reason why they didn't but we don't know.

I was watching the "Operation Think Tank" video on The Chieftain's YouTube channel and got an insight.  My current impression is that with the short life of individual tanks and the speed of development in WWII that it did not occur to the planners that M4 Sherman 76mm tanks would be around very long.  It probably did not seem to be worth the trouble to develop a new HE shell and associated manufacturing line.  Besides, the M4 model carrying a 105mm howitzer was accepted into service at virtually the same time and it would be a much better HE thrower.    The new M26 Pershing tanks with their 90mm guns would soon be available and be better hole-punchers.   

According to this presenter, there were some plans in late 1945 for postwar units to have mainly Sherman M4's fitted with 105mm howitzers to dole out HE and a much smaller percentage of M26 Pershings to deal with other tanks.

https://youtu.be/C1sN52CBKi4?list=PLEAEU2gs2Nz-aSi3PpjNI9Q4klDGi421D&t=1199